## Double Slit III: Delayed Choice

Interesting physics explained with many thought experiments and little math.
SsDd
Science Officer
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AKA: Abhi
Location: Michigan Tech

### Double Slit III: Delayed Choice

The lecture video is embedded below.

Additionally, slides used in the lecture are embedded below, or can also be downloaded directly from here.

Questions after the lecture? Please feel free to post them in the same thread.

Click to play embedded YouTube video.

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Double-slit experiment
Wave–particle duality

maplebayou1
Ensign
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Joined: Tue Oct 05, 2010 9:45 pm

### Re: Double Slit III: Delayed Choice

I think the strangeness of QM is that it really seems to violate classical ideas of cause and effect. In classical mechanics, we can ask a question like, "Why does an object accelerate the way it does?" We will get the answer that it is because it has a certain mass and a certain force is applied to it (ignoring relativity). There is a simple mathematical relationship and we never have to deal with anything coming after the acceleration when we do the calculation. But in QM, we can have an apparatus which "causes" which-path info to be available interacting with a particle long after its entangled twin is obliterated. To correctly predict (postdict?) what the entangled twin will do/has done, we need to know what happens later. It certainly seems as though, in a sense, cause and effect are reversed in time. At the very least we cannot really describe what the particles are doing (if they are "doing" anything before observed) without recourse to a kind of time-reversed (or time-neutral) causality.

Samuel Andrews
Asternaut
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Joined: Mon Jul 29, 2019 9:28 am

### Re: Double Slit III: Delayed Choice

maplebayou1 wrote:
Wed Oct 06, 2010 1:14 am
I think the strangeness of QM is that it really seems to violate classical ideas of cause and effect. In classical mechanics, we can ask a question like, "Why does an object accelerate the way it does?" We will get the answer that it is because it has a certain mass and a certain force is applied to it (ignoring relativity). There is a simple mathematical relationship and we never have to deal with anything coming after the acceleration when we do the calculation. But in QM, we can have an apparatus which "causes" which-path info to be available interacting with a particle long after its entangled twin is obliterated. To correctly predict (postdict?) what the entangled twin will do/has done, we need to know what happens later. It certainly seems as though, in a sense, cause and effect are reversed in time. At the very least we cannot really describe what the particles are doing (if they are "doing" anything before observed) without recourse to a kind of time-reversed (or time-neutral) causality.
you r right